The invention is more particularly but not exclusively applicable to radio communication systems like those described in French patent No. 2 777 407 submitted on Apr. 10, 1998 under the name of the same applicant as this patent application and entitled “cellular radiotelephony signal with additional channel assigned to the down direction, and corresponding method, system, mobile and base station”.
Such systems comprise a two-directional symmetric main channel comprising a main up channel (from a mobile terminal to a base station) and a main down channel (from a base station to a mobile terminal) particularly for the transmission of signalling and control data and information at low or medium speed. They also comprise an additional channel assigned to the down direction, which carries high speed data transmission. This additional channel preferably uses a multicarrier modulation, for example of the OFDM (Orthogonal Frequency Division Multiplexing) type.
More generally, the invention is applicable to any radio communication system using data transmission by multicarrier modulation with distributed pilots, in which a receiver has information (direct or indirect) about the source of the multicarrier modulated data that it receives.
Its applications are particularly in the context of the 3.5G mobile radio communication system in which a down link called an HSDPA has high speed and mobility characteristics and can use an OFDM type multicarrier modulation.
Remember that a multicarrier modulation is a digital modulation, in other words it is a method of generating an electromagnetic signal from digital information to be transmitted. The innovation and the advantage of such a modulation is to breakdown the frequency band allocated to the signal into a plurality of sub-bands chosen to be less wide than the channel coherence band (in other words the band for which the frequency response of the channel may be considered to be correlated on a given duration) and on which the channel can therefore be assumed to be constant during the symbol transmission time. The digital information to be transmitted during this period is then distributed on each sub-band, so as to:                reduce the modulation speed (in other words increase the symbol duration) without modifying the transmitted speed;        simply model the action of the channel on each sub-band, using the complex multiplier model.        
In reception, a simple received data correction system (consisting of making a complex division using the estimated channel) provides a means for satisfactory recovering information sent on each carrier, except for carriers on which deep fading has taken place. In this case, if no information protection measures are taken, data transported by these carriers will be lost. Therefore, a multicarrier system is only interesting if the generation of the electric signal is preceded by digital data processing, for example such as an error correction coding and/or interlacing.
Recovery of the information sent on each carrier in reception is conventionally preceded by a channel estimating step during which the transfer function of the transmission channel is estimated.
When the channel can be considered to be practically constant on each sub-carrier (multiplication channel), it can then be modelled by a complex estimation coefficient Hm,n (where m is the index of the sub-carrier and n is the index of the OFDM symbol considered).
The invention is more particularly applicable to the case in which the channel estimate is made using pilots distributed within useful data elements to be transmitted, in the time-frequency space.
The channel estimate by distributed pilots consists of inserting reference carriers in the flow of useful carriers, at locations known to the receiver. In reception, the values taken on by these reference carriers called pilots are read, and the complex gain of the channel is easily deduced at these reference locations. The complex gain of the channel is then deduced on all points of the transmitted time-frequency network, from the calculated value of the complex gain at the reference locations.
All emitters in radiomobile systems using an OFDM modulation known at the present time share the same time/frequency resources: in other words, all emitters can emit data to at any known instant and can use the entire frequency band allocated to the system. Useful data elements are distributed in the time-frequency plane used, and the pilots are inserted between these data elements according to a predetermined distribution known in emission and in reception, called a pilot pattern.
All emitters in all these systems, for example such as HiperLAN, IEEE802.11, or DAB, use the same pilot pattern common to the entire system.
Therefore one disadvantage of these conventional systems is that interference between pilots emitted by distinct but geographically close emitters is inevitable. Since all emitters use the same pilot pattern, several pilots are emitted at the same time and at the same frequency by the different emitters in the system, such that any one receiver receives the pilot emitted by the emitter associated with it, and also one or several interfering pilots originating from one or several neighbouring emitters.
Therefore the channel estimate in reception and extraction of the corresponding useful information are strongly disturbed by the existence of this inter-pilot interference. Therefore the reception quality and performances of the receiver are strongly reduced.
At the moment, no known radio communication system has used a technique capable of cancelling or at least reducing this harmful interference phenomenon, due to the a priori very high complexity of such a technique.